A light waveguide is formed in a birefringent crystalline substrate such as lithium niobate by depositing a metal film followed by heating of the substrate in a flowing inert gas such as argon, nitrogen or oxygen at about 1,000.degree. C. This results in a metal diffused layer with a higher refractive index near the surface. The metal film is typically selected from vanadium, titanium, nickel, and copper. Of these metals, titanium-diffused lithium niobate waveguides have exhibited preferred optical properties for many applications.
Though this type of surface channel waveguide can guide propagation modes efficiently, insertion loss is high because of surface scattering and mode mismatch between the waveguide and a coupled fiber. Propagation modes of such surface channel waveguides are inherently asymmetric with respect to the guiding axis. By burying the waveguide under the substrate surface, modes can be made more symmetric and better matched with the fiber. Magnesium oxide has been found useful for burying the waveguide. Both the ordinary and extraordinary refractive indices of lithium niobate have been found to be reduced by diffusion of magnesium. However, deposition of magnesium in lithium niobate is difficult as special precautions are required. Proton exchange in lithium niobate has also been used to bury waveguides below the surface; however, with proton exchange only one polarization mode can be guided by the waveguide.
A polarization beam splitter is a device that separates an incoming beam of light into orthogonal TE and TM modes at the output ports. Several approaches are known for achieving a polarization splitter, including mode sorting and modal interference. Other techniques include use of a directional coupler with metal loading on one of the waveguides to prevent the TM mode from coupling. Another approach employs electro-optical effects acting differently on TE and TM coupling lengths, allowing both to be tuned independently. However, most of these techniques require stringent fabrication tolerances.
A continuing need exists to provide improvements in optical waveguides to provide more efficient coupling of light and ease of manufacture.